Hydrophobic mats for gas diffusion electrodes

ABSTRACT

A GAS DIFFUSION ELECTRODE HAVING A GAS ENTRANCE SIDE AND AN ELECTROLYTE CONTACTING SIDE, FOR USE WITH A LIQUID ELECTROLYTE AND A GAS IN AN ELECTROCHEMICAL CELL, COMPRISES A COHERENT POROUS CATALYZED BODY, CONTAINING AN ELECTRICAL CONDUCTOR AND A HYDROPHOBIC OUTER LAYER ON THE GAS ENTRANCE SIDE. THE HYDROPHOBIC OUTER LAYER COMPRISING A MAT CONTAINING FIBRILLATED VERY HIGH MOLECULAR WEIGHT POLYETHYLENE.

March 26, 974 R N SAMPSON ET AL HYDROPHOB'IC MATS FOR GASDIFFUSIONVELECTRODES Filed Aug. 5l, 1972 SCRUBBED 3 Am AIR 3,799,811HYDROPHOBIC MATS FOR GAS DIFFUSION ELECTRODES Ronald N. Sampson,Murrysville, and Jacob Chottiner,

McKeesport, Pa., assignors to Westinghouse Electric Corporation,Pittsburgh, Pa.

" Filed Aug. 31, 1972, Ser. No. 285,164

Int. Cl. H01m 13/00 U.S. Cl. 136-120 FC 6 Claims ABSTRACT OF THEDISCLOSURE A gas diffusion electrode having a gas entrance side and anelectrolyte contacting side, for use with a liquid electrolyte and a gasin an electrochemical cell, comprises a coherent porous catalyzed body,containing an electrical conductor and a hydrophobic outer layer on thegas entrance side. The hydrophobic outer layer comprising a matcontaining brillatedlvery high molecular weight polyethylene.

BACKGROUND OF THE INVENTION This invention relates to electrochemicalcells, such as fuel cells or hybrid metal-gas cells, and moreparticularly it pertains to electrodes for such cells which have a newand improved hydrophobic layer comprising a mat containing brillatedvery high molecular weight polyethylene.

=Fuel cells are electrochemical devices which convert the chemicalenergy in a fuel directly into electrical energy by the oxidation offuel supplied to the cell. The fuel cell is composed of two gas diiusionelectrodes adjacent to and in contact with an electrolyte, with meansfor supplying a fuel to one electrode and an oxidant to the otherelectrode. In a gas diffusion electrode, the gas penetrates by diffusionto a three-phase zone, which is a narrow electrochemically active zonelwhere the gas,

kliquid electrolyte, and the solid particles of the electrode meet. Acatalyst is usually used to accelerate the electrode nited States PatentO ice trode structure, and the sheets may lose as much as 80% of theiroriginal air permeability during this heat pressing step, which isgenerally required in air electrode manufacture.

The use of porous polyethylene films has been suggested as ahydrophobic'membrane for fuel cell electrodes, by Moos in U.S. Pat.3,097,116, in conjunction with a catalyzed synthetic zeolite(Na2O.Al2O3.(SiO2)2 or K2O.AlO3.(SiO2)2) active layer. The use ofconventional polyethlyene polymers, i.e. those having weightaveragemolecular weights of about l00,000-400,000 and melt ow indices of about1-70, however, provides a melted low air permeable ilm at the processingtemperatures encountered in air electrode manufacture. Shouldhydrophobic ilms prepared from conventional polyethylene polymers retainsuflicient air permeability for use on an air electrode, they often arefound to have reduced hydrophobicity as a result of required pressing athigh temperatures in air electrode manufacture.

What is needed is an expensive hydrophobic membrane for gas diffusionelectrodes, having the ability to retain hydrophobic and airpermeability qualities after pressure and heat bonding to the activeelectrode layer in the electrode manufacturing process.

SUMMARY OF THE INVENTION The above need is met by making an airpermeable hydrophobic membrane mat, having a heat and compressionresistant mat structure, from a blend of very high molecular weightpolyethylene polymers, having a low melt ow index, and thermoplasticmaterials that are extractable from the blend by a solvent extractionprocess.

The gas diffusion electrode of this invention in one of its preferredforms comprises a catalytic layer contained within a porous electricalconductor plaque. The catalytic reaction in gas electrodes. Ideally, thecatalyst is most l effective when it is located at the active interfacewhere the electrolyte and gas meet in the presence of an electricalconductor. Preferably, that interface is close to the gas phase so thatthere is a short diffusion path for the gas.

A gas ditfusion electrode is also used in hybrid batteries. In these,the dilusion electrode is fed with air or oxygen and is generally pairedwith a metal electrode. In operation, the chemical energy of oxidationof the fuel or of the metal is converted into clerical energy.

A common weakness of gas diffusion eleoctrodes is the occurrence ofsweating or weeping which is the formation of tiny drops of electrolyteon the gas side of the electrode due to penetration of the electrolyte.A

variety of methods have been tried to attempt to avoid penetraion of theelectrode by the electrolyte.

The prior art has recognized and sought to alleviate the problem ofelectrolyte penetration through the electrode. Fluorocarbon polymers,used to prevent this penetration as binders to ill the electrode poresand as pure polymeric sheets in dual structure electrodes, are describedin U.S. Pat. No. 3,385,780. Fluorinated polymer, coated on glass clothsubstrates-as a barrier sheet in liqudi-liquid type fuel cells for thepurpose of preventing liquid oxidant from reacting with catalyst in theelectrode, are described in U.S. Pat. No. 3,382,103.

Fluorocarbon polymers in the forms employed within the air electrode andas a barrier sheet are very expensive and may cost as much as 3 to 15dollars per pound. Also, high pressing temperatures, on the order of 190C,. are required to bond these sheets to the active air eleclayercontains particles of electrically conductive material inert to theelectrolyte, such as carbon, boron carbide, graphite, other carbonaceousmaterials inert to the electrolyte, or nely divided metals, and aresinous binder inert to the electrolyte, such as polytetrauoroethylene.The catalytic layer includes a catalyst, such as platinum. The catalyticlayer will contain suliicient binder to provide partial wet proofing.'Ihe high molecular weight polyethylene hydrophobic layer of thisinvention is rolled or hot pressed to one side of the partially wetproofed electrode. This results in a gas diffusion electrode providingimproved periods of dry operation while retaining excellent airpermeability.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of theinvention, reference may be made to the preferred embodiments, exemplaryof the invention, shown in the accompanying drawings in which: Y

FIG. 1 is a schematic view through one embodiment of a gas diffusionelectrode, showing the hydrophobic wet proof barrier layer;

IFIG. 2 is an enlarged view of the surface of the hydrophobic membranemat of this invention, showing the fbrillated, very high molecularweight polyethylene f -i trode mounted in hybrid battery; and

strands, the irregular shaped high molecular Weight polyethyleneparticles and the optional interlocking reinforcement iiller material;

FIG. 3 is a schematic view showing a gas diusion elec- FIG. 4 shows anelectrode testing device. y

' FIG. 1 vschematically illustrates onev embodiment of an electrode 10,comprising a coherent porous body having withlthree layers includinganfoptional'backing or 'sup` porting layer 11 next to electrolyte 12.The necessary layers include gas entrance catalytic layer 13 and, gaspermeable wet proof hydrophobic mat barrier layer 14. The layers 11 and13 are shown on opposite sides of a porous electrical conductor plaqueor grid 15.

The catalytic layer 13 can be composed of particles: of a carbonaceousmaterial selected from the group consisting of carbon, graphite, boroncarbide, and mixturesr thereof. When carbon is used as the conductingmaterial, the particles have a surface area of from about 5 to 1000square meters per gram. In addition, the layer 13l includesl a binderinert to the electrolyte, for example, polymers and copolymers ofpolysulfone resin, polyethyleneresin, polypropylene resin, or afluorocarbon or chloro-uorinated hydrocarbon polymer, that binds theparticles of conducting material together'in a porous manner. The amountof binder may vary' from about 10 to 50 weight percent of the totalcomposition of layer 13,'with a preferred range of from about 20 to 50weight percent.-

In addition, layer 13 includes a suitable catalyst which' would containat least one of the metals of a group consisting of the platinum groupmetals (Pt, Ir, Ru, Rh, Pd),V mercury, gold and silver. Other catalystsmay be used." Their choice depends on the reaction proceeding at theelectrode. 'I'hus for the oxygen electrode, members 'of the transitionelements or mixtures thereof, or silver, gold or their mixtures withoxides, nitrates, etc. are examples f eicient catalysts. The catalyticmaterial is generally added to give active catalyst in an amount varyingfrom about 0.1 to 10 milligrams per sq. cm. of geometric electrode area.i

The optional supporting layer 11, when used, is com-y posed of particlesof a carbonaceous material and binder similar to that used in layer 13,but the amount of binderY may vary from about to 50 weight percent ofthe total composition of the layer. The supporting layer although addingstrength to the structure may be eliminated if space considerations areimportant, or if the electrode" structure is strong enough to eliminateits need, as whenbonded fibrous metal wool members are used-'as'theelectrical conductor 15. A typical thickess of the electrode may varyfrom about 20-65 mils.

The porous metal electrical conductor 1S maybe a wire mesh membenanexpanded metal membexja perforated metal sheet,'or'preferably, a fibrousmetalwool member that is diffusion bonded at 'points of'iiber contact.The conductor is generally composed of nickel-,"ste'el or iron that maybe plated with a noble metal when used as',` an air electrode. When ironor steel wool is usedasthe`` electrical conductor it can be coated withnickel' 'a'ndfivillv provide a conductor 15 having a bondedmatrixfofbe'r's' about 0.01-0.04 inch thick, into which themateriaLconstituting the catalyzed layer 13 can be intruded by a Wetpasting or other technique. v j Y 'The gas permeable hydrophobic barrierlayery membrane mat 14 is shown in an enlarged rview in' FIGZ.YThefm'at',

comprises brillated strands 17 of very high molecular" weightpolyethylene, which'are interlocked andpressed together along withirregular shaped particles Y1"8"o'i: "tvery in diameter, with an averagepore size of about 20 microns.

To provide additional strength for the barrier layer mat 14, large,interlocking filler material libers 19 can be uniformly distributedthrough the mat. The reinforcement, when used, should be of a relativelynon-wettable thermoplastic material such as, for example, uorocarbon,polyamide, polyester, polypropylene or polyethylene fibers, about 0.01to` 2 inches long, preferably about 0.06-2 inches long, and about00005-005 :inch in diameter.

The polyethylene polymer is essentially a long chain aliphatichydrocarbon of the type The polymers that function as the usefulhydrophobic air permeable membrane mat, for the electrode for thisinvention, are'very, high molecular weight polyethylene polymers, havinga Weight-average molecular weight (Mw) over 1,750,000 and up to about5,000,000 and a melt flow index (grams of a thermoplastic which can beforced through a 0.0825 inch orifice at 2160 grams force in 10 minutes)up to about 0.02. These polymers are made of very long,nearly branchfree chains and have a density of about 0.`940.96 gm./cu. cm. Thepolymer may be made, for example, by coordination polymerization withZiegler catalysts, as described in Brydson, Plastics Materials (1966),Chapter 7. This type polymer produces tough, resilient strandswhensheared in particulate form, which enable the'membrane mat -toresist melt flow when the membrane is laminated to the electrode body.

In the method of this invention an air permeable hydrophobic mat,suitable for use as a barrier layer on an air electrode is made by (1)mixing: about 4 to 35 weight percent high 'molecular weightpolyethylene, having a weight'average molecular weight of over 1,750,000and a melt flow index Aof up to about 0.02; about 40 to 80 weightpercent of at least one solvent-extractable highly viscous'lthermoplastic polymer'such as, for example, polycarbonatepolymer;polymethylmethacrylatepolymer, polystyrene polymer',polyethylene oxide polymer, and the like with polymethylmethacrylate andpolystyrenev preferred; about 0fv to 40 weight percent but, preferablyabout 15 to 40 weight percent, of a solvent"extractable plasticizer,

" effectivev to` improve' the flowand extrusion properties ofnon-water-Wettable thermoplastic reinforcement lbers;

and about 0 to 5 weight percent fluorocarbon particlessuch aspolytetrali'uoroethylene; the weightpercent referring'to the total'blend after complete'mixing, (2) fibrillatin'g'at least about 15 percentof the high density, very high rr'ilelarweight-polyethylene, '(3)forming a sheet from'.

the blend'and (4) extracting the solvent-extractable polymer andplasticizer"from the'sheet with asuitaible' solvent fora'-tinieeffective to ldissolve the extactable vpolymer and plasticizerbut' not the highv density polyethylene and thermoplastic 'supportingi'ibersj' constituting the mat, to

about 0.5 to 4000 microns long and about 0.00`5I5to 40" microns indiameter. The particles are between about- 0.05 to 400 microns indiameter. The high molecular vweight polyethylene strands andparticlesprovide 'a'very tough, resilient, heat resistant, interlockedmat, having; about 20-50% porosity and a pore size, shown as 20, thediameter circle that can be inscribed between strands and particles,ranging from about 0.003 to about 100 microns provide an fair"permeable, hydrophobic' mat vcomprising brillated, interlocked,"highdensity, lhigh molecular weight poly,etlz'ififlene.v Y f More"speciically,'high density, very high molecular weight vpolyethyleneparticles, having an average particlel size "up Vto about-100 microns indiameter, are dry blended atf'abut 25` C. -witha solvent extractablepolymer-'arid preferably; `a solvent extract'able plasticizer that isthermally "stable at the processing temperature and com- "lpatible"'with`"the solvent extract'able polymer. lSuitableplasticzersfforthe solvent' extractable polymers disclosed are knowntothose in theart, described inthe Plasticizers Ch'a'r't'i '1968 ModernPlastics' Encyclopedia, pp. 460- n 472,-andfgenerally include phthalicacid derivatives, benzoic acidderiva'tives, and adipic acid derivatives.vAfter about l30inni'rilltes thisv mixtureis placedjonto a hot two rollmill, having rollers rotating toward each other and spaced generallybetween about 0.005-0.1 inch apart, depending on the batch size. Themill is operated at a temperature between about 85225 C., preferably 85-135 C., and is effective to soften or melt the solvent extractablepolymer and shear-fibrillate the high molecular weight polyethyleneparticles i.e. the particles are deformed by the stresses into a long,thin strand or liber shape. The mixture, is worked into a soft, viscous,rubbery composition that is distributed between the two rolls. f Afterabout 2 minutes, thermoplastic reinforcement bers and/or an aqueousdispersion of polytetrauoroethylene particles, preferably having anaverage particle size of about 0.05- microns in diameter, may optionallybe added to the admixture in the hot roll mill.

After about 8-16 minutes on the hot roll mill or other suitable mixingmeans, which can act to heat-mix and shear-brillate, the blend in sheetform is removed and cooled. Preferably, the blend layers on the rolls inthe hot roll mill repeatedly cut from the rolls as a sheet and then fedback between the rolls at a different angle.

The blendLmay then optionally be broken into par ticulate form,generally in a hammer mill and may then be placed in a heated screw orplunger type extruder, and forced through a die to furthershear-brillate the particles in the blend. Preferably, a at tape diebetween about 0.010.02 inch is used. This step while providing astronger membrane mat is optional.

The sheet is then placed in a molding frame and molded in a press forabout 2-10 minutes at about 125 180 and 300-2000 p.s.i., and then cooledto -50 C. under pressure to form a sheet about 0.02-0.06 inch thick.

The solvent extractable polymer and plasticizer are then extracted fromthe sheet to form an air permeable hydrophobic mat. Preferably the sheetis placed between bronze screens and immersed in a stirred bath ofsolvent Which is effective to dissolve the extractable components in thesheet but will not appreciably attack or dissolve the other componentsmaking up the membrane mat. Examples of some suitable extracting agentsare acetone, toluene, methylene chloride, methyl ethyl ketone,cyclohexanone, and diacetone alcohol among others. Naturally, a solventwill not be used which will harm the interlocking polyethylene or thesupport fibers of the mat. Preferably, the sheet is placed in an acetonebath for about 16 hours and then placed in a fresh acetone bath foranother 2-3 hours, after which it is placed in a deionized water bathand then removed and dried. The mat at this point will be between about0.0100.045 inch thick, about 20-50% porous, and comprise unmelted,pressed, interlocked, brillated strands and irregular shaped particlesof high density, very high molecular weight polyethylene, with uniformlyscattered thermoplastic reinforcement fibers and brillated fluorocarbonparticles if such were added to the admixture.

l In this process it is essential that the high density, very highmolecular weight polyethylene, which is initially mixed with the othercomponents have an average particle size no greater than about 400microns, or it will be difficult through shearing forces to achieve the15-85 percent fibrillation desired. If temperatures much over 225 C. areused n the mixing and fibrillation step the system will generally be toofluid, shear forces will be reduced and it will be difficult to achievethe degree of fibrillation desired. The fibrillation is desired, so thatthe mat will have a fibrous, paper-like structure after the extractionstep. A structure comprising pressed, interlocked, fibrillated strandsis advantageous in providing an open structure which will act in aresilient fashion during hot bonding to the active electrode body, so asto retain its air permeability qualities.

The mat is thenbonded onto the gas diffusion electrode, which ispreferably a fiber metal matrix with an intruded catalytic layer, toform an air permeable, hydrophobic composite. This bonding step iscarried out at about -200 C., preferably by placing the mat on thecatalyzed layer and passing the mat and electrode through heatedlaminating rolls, at between about 170- 375 lb./lineal inch of electrodewhich heat-press the mat onto the electrode without melting the highdensity, very high molecular weight polyethylene in the mat. In thisprocess it is essential that the high density polyethylene have a weightaverage molecular weight over 1,750,000 and a melt oW index below about0.02, so that the strands and irregular particles comprising the matwill not flow and compact into a low porosity melted film or sheetduring heat bonding to the gas diiusion electrode.

By using a fbrillated structure and high density, very high molecularweight, low ow polyethylene, the air permeability of the mat, afterheat-pressing the mat to the electrode, is only reduced between about30-60%, versus about 80% reduction in permeability for pure Teonmembranes, and essentially for conventional polyethylene sheets(Mw=l00,000-500,000 and melt ow index=1-70). The conventionalpolyethylene sheets generally weld into a non-porous form attemperatures and pressures required to bond the barrier layer to theelectrode structure. In actual use as an air electrode in a metal-airbattery, the electrode 10- may be employed as shown in FIG. 3. For thatpurpose the electrode 10 is mounted between a pair of frame members 21and 22 which are disposed between end plates 23 and 24. An air chamber25 is provided between the end plate 23 and the electrode 10. Likewise,a chamber 26 is provided between the electrode 10 and the end plate 24,which chamber is filled with electrolyte 27 such as a 25-30 weightpercent solution of NaOH or KOH. An electrode 28 and a chargingelectrode 29 (for recharging the battery) are disposed in the chamber 26and within the electrolyte. The electrode 28 is composed of an oxidzablemetal such as iron, cadmium or zinc. The charging electrode 29 iscomposed of an inert metal such as nickel. The electrode 28 is encasedin an envelope 30 having an open top 31. The envelope 30 serves as aseparator consisting of a sheet of cellophane sandwiched between sheetsof fibrous polypropylene. The oxygen electrode is positive with respectto the metal electrode. When charging, the charging electrode ispositive with respect to the metal electrode. A vent 32 in the framemember 22 is provided to permit the escape of gases from the electrodes28 and 29 when charging. Wire leads 33, 34 and 35 extend from theelectrodes 10, 28 and 29, respectively. An air inlet 36 and an airoutlet 37 are provided in the end plate 23.

The electrode 10 was tested against an inert counterelectrode 40 in adriven circuit, such as shown in FIG. 4, for which purpose it was placedin an electrode holder 41, in conjunction with a reference electrode 42.As shown in FIG. 4 the assembly of the electrode holder 41 and theelectrode 10 is immersed in an electrolyte 43, such as aqueous KOH,contained in a container 44. A counter electrode 40, composed of a metalmesh such as platinum or nickel, is likewise immersed in the electrolyte43. The cell including the electrodes 10 and 40 in the electrolyte 43 isdriven by a 12 volt battery 45 for testing with the electrode 10connected to the circuit by a lead wire 46, which extends between theinterfaces of the frame member 47 and the portions 48, and which isconnected to the upper end of the grid conductor 15. The electrodeholder 41 is provided with an inlet tube 49 and an outlet tube 40 whichcommunicates with the portion of the opening 51 between the plateportion 52 and the electrode 10, whereby the active gas such as oxygenis in contact with the catalyzed gas entrance layer 13 and the barriersheet layer 14.

The reference electrode 42 is used in conjunction with a Luggincapillary having an opening 53 which is located two mm. from the surfaceof the electrode 10, in order to measure the potential of the electrodeagainst a point in the electrolyte located as closely as possible to theelectrode 10. The electrode 42 includes a mercury/mercury oxide mixture54 located in a glass bulb 55 that communicates via an inverted U-shapedglass tube 56 with the Lugging capillary opening 53 on the electrolyteside of the electrode 10. The tube 56 is -lled with electrolyte 43. Thetube 56 is U-shaped to facilitate attachment of the electrode 42 and theelectrode holder 41. A platinum wire -60 leads from the Hg/HgO mixture54 to one side of a high impedance voltmeter 61, the other side of whichis connected to the electrode 10. When air is used as an active gas andthe electrolyte is alkaline (KOH), the air before entering the devicemay be scrubbed by passing it through an alkaline solid absorbent or analkaline solution.

EXAMPLE. 1

Mats comprising iibrillated very high molecular weight polyethylene weremade by the following procedure. A mixture of 40 grams (10 wt. percent)of very high molecular weight polyethylene powder, having an averageparticle size of between about 10U-300 microns (140-50 mesh size-U.S.Screen No.), a crystalline melting point of 130-131 C., a waterabsorption factor (ASTM designation D570) of 0.03%, a density at 20 C.of 0.940- 942 gm./cu. cm., a weight average molecular weight of about2,000,000 (Mw) and a melt flow index (ASTM designation Dl238) of nili.e., about 0.01 gr./l min. (sold commercially by Hercules Powder Co.under the trade name Hi-Fax 1901 Shock and Abrasion ResistantPolyethylene); 320 grams (80 wt. percent) of polymethylmethacrylatebeads, having an average particle size of about 400 microns; and 32grams (8 wt. percent of the total blend) of separated, fibrouspolytetratluoroethylene iller, 0.016 inch long lloc, was placed in aPatterson- Kelley twin shell dry blender and mixed for 45 minutes atroom temperature.

This mixture was then added to a Farrell two-roll mill (6 inches dia.rolls by 13 inches long, about 0.04 inch apart maintained at atemperature of 180 C.) The mixture milled for 2 minutes to obtain aviscous batch on the rolls, then 7 grams (2 wt. percent) ofpolytetrafluoroethylene particles, about 0.05 micron particles size on a60% solids aqueous dispersion (sold by Du Pont-under the trade nameTeflon 30B Emulsion) was added to the batch and the milling wascontinued for minutes at a roll speed of about 35 ft./min., to ash oithe water and form a viscous, rubbery blend.

The batch was cut from the rolls, in sheet form, rotated 90 and fed backinto the nip between the hot rolls. The batch was milled again for 5minutes at about 35 ft./min. and 180 C. and then cut from the rolls inthe form of a relatively stil sheet about 0.04 inch thick. This drymixing and hot roll milling operation mixes the components of the blendand iibrillates the polyethylene and polytetrafluoroethylene particlesby the shear forces exerted by the hot rolls. If the rolls aremaintained at a temperature much over about 225 C., the blend becomestoo uid and the shear forces and fibrillation are reduced. It the highmolecular weight polyethylene powder has an average particle size overabout 400 microns, it will be diflicult to convert to the thintbrillated strands desired for good interlocking in the iinal mat. Theuse of a suitable, extractable plasticizer would effectively modify themilling and extrusion properties of the blend, provide a more llexiblesheet olf the hot rolls and allow use of lower temperatures in the hotmilling-fibrillating step.

The-sheet of blended material was thenv cooled and placed in a Wyliehammer mill to grind up the sheet and provide a particulate blend havingan average particle size of about 0.07 inch (10 mesh size). The groundblend was then divided into two portions, Sample A and Sarnple B.

Sample A was placed in a Wayne extruder and extruded through a 0.010inch tlat tape die, to further fibril- Alate the components of theblend. The extruded tape was then cooled and reground in a Wylie hammermill, as described above, to provide about 10 mesh average sizeparticles.

Sample A and Sample B were then placed in a stainless steel moldingframe about 6" x 6" x 0.050", and heat-pressed at C., and 1500 p.s.i.for about 10 minutes in a Elmes Hydrocure iiat-bed molding press. Eachmolded sheet was cooled to less than 50 C. under pressure in the pressbefore being removed.

The Sample A and Sample B sheets were placed between bronze screens andimmersed in acetone for about 16 hours to extract the solventextractable polymethyl meth--l acrylate from the sheet. The sheets werethen immersed in a fresh stirred acetone bath for another 4 hours,removed, rinsed in deionized water and dried. The acetone did not harmthe polyethylene fibers and particles of the fluorocarbon filler bers.

The sample both had good integrity and exibility and were both insubstantially non-wettable mat form. Under 150x magnilication they wereseen to comprise pressed,

interlocked thin strands and irregular shaped particles disperseduniformly through the mat and forming the matrix of the mat, with longerinterlocking supporting fibers uniformly dispersed through the matrix.Porosity for both samples was between about 25-50% and the pore size wasabout 0.003-100 microns in diameter between the bers and particles.Sample A was stronger than Sample B and appeared to have morevibrillated strands, a ratio of about 3 strands to 7 particles (about 30%fibrillation) as compared to Sample B, about 2 strands to 8 particles(about 20% brillation), due primarily to the extrusion of the Sample Ablend. Both samples had a paper-like, rather than smooth finish andlooked porous when held up to the light.

Sample A and B mats were tested for air permeability by testing 3.25 x2.75" pieces. The air permeability apparatus consisted of a frame intowhich the mat was clamped, a llowmeter to measure the air flow rate, ananometer for measuring the pressure differential across the cut matsand a vacuum source. The cut mats were placed in the metal clamp frameand checked for. air tightness by sealing off the air intake part of theframe. The intake part was opened and the pressure differential acrossthe cut mat pieces adjusted to 6 inches of water by throttling thevacuum. The air flow as measured by'the owrneter was then recorded. Theresults were as follows:

Air permeability Thickness Density (ou. cm./ Sample (mils) (gm/mil)min.)

A (extruded) 34 0.077 375 B 32 0.083` 370 Sample A and B mats were thenheat-pressed by passing them through hot rolls at `Cfand200-350`lb'l7lineal inch to simulate the conditions required'inbondingfhydrophobie barrier layers to form a composite*gas'diffusionlelectrode. The mats were thenl retested `for permea- Iity: n

v'.Air permeability f Thickness Densit (cu. em./ Sample (mils) (gmJmilmin.) Y,

A (extruded) 29 v 0. 09o 225, B 25 0. 107

and particles in the mats were pressed rather than melted together andafter compression there appeared to be no melting or flow, the structuremaintaining its porosity. This method provided a non-wettable, porous,air permeable barrier layer membrane made from polyethylene costingabout lifty cents per pound, thus providing a distinct cost advantageover polytetralluoroethylene membranes.

EXAMPLE 2 In this example, two films, one made frompolytetrafluoroethylene, Sample C, and one made from regular lowmolecular weight polyethylene, Sample D, were made for comparativepurposes. In the case of Sample C, the blend comprised about wt. percentof polytetrauoroethylene particles having an average particle size ofbetween about 0.05-0.5 micron about 50 wt. percent ofpolymethylmethacrylate thermoplastic and about 40 wt. percent ofdicyclohexylphthalate solvent extractable plasticizer. In the case ofthe Sample B, the blend comprised about wt. percent of polyethyelnepowder, having an average particle size of between about 100-300microns, a melt flow index of about 8.0, and a weight average molecularweight of about 400,000 (Mw); about 42 wt. percent ofpolymethylmethacrylate thermoplastic having an average particle size ofabout 400 microns and about 28 wt. percent of dicyclohexylphthalatesolvent extractable plasticizer.

Both Samples C and D were pre-blended and hot roll milled as in Example1, under the same process conditions except that the rolls weremaintained at about 110 C. Sheets about 0.040 inch thick for bothsamples were cut from the rolls and each placed into stainless steelmolding frames about 6" x 6" x 0.030".

Sample C and D sheets were then heat-pressed at 150 C. and 600 p.s.i.for about 5 minutes in a molding press and then cooled under pressurebefore being removed. The Sample C and Sample D sheets were then placedin separate extracting baths, as in Example 1, under the same processconditions.

The sample D sheet had good integrity but was translucent and whenexamined under a 150)( power microscope showed no signs of brillatedpolyethlene. Sample C had good integrity and flexibility and wassubstantially non-wettable, about 40% porous, and about 60% fibrillated.Sample D was very glossy and smooth and looked very much like athermoplastic ilm extruded by a melt extrusion process. There hadapparently been extensive melting in the Sample D sheet. The Sample Dsheet was quite hydrophobic as determined by the non-wetting of waterdrops placed on the surface.

Samples C and D were tested for air permeability by the method used inExample l and Sample C was then heat-pressed as in Example 1. Theresults were as follows:

.Air permeability Thickness (mils) (eu. em./min.)

Before After Before Alter comprescomprescomprescompres- Sample sion sionsion sion C (Teon) 16 210 42 D (low mol. wt. polyethylene) 5 EXAMPLE 3Two mats, Sample E and Sample F, comprising high molecular weightpolyethylene were made, bonded 4to the active layer of a gas diffusionelectrode to` vform an air permeable hydrophobic composite, and tested.

Sample E A mixture of 21 grams (8.4 wt. percent).of very high molecularweight polyethylene powder, having an average particle size of betweenabout 100-300 microns, a crystalline melting point of 130-131 C., awater absorption factor of 0.03%, a density at 20 C. of 0.940-0942gm./cu. cm., a weight average molecular weght of about 2,000,000 and amelt .flow index of nil i.e., about 0.01 gr./ 10 min. (Hi-Fax 1901); 128grams (51 wt. percent) of polymethylmethacrylate beads, having anaverage particle size of about 400 microns; and grams (34 wt. percent)of dicyclohexylphthalate plasticizer; was placed in a Patterson-Kelleytwin shell dry blender for 45 minutes at room temperature.

This mixture was then added to a Farrel two-roll mill (6 dia. rolls b13" long, about 0.0 apart maintained i Sample F A mixture of 21.5 grams(4.3 wt. percent) of very high molecular weight polyethylene ilake,having an average particle size of between about -300 microns, acrystalline melting point of -131 C., a water absorption factor of0.03%, a density at 20 C. of 0.940-0.942 gm./cu. cm., a weight averagemolecular weight of about 2,000,000 and a melt flow index of nil, i.e.,about 0.01 gr./l0 min. (Hi-Fax 1901); 225 grams (51 wt. percent) ofpolymethylmethacryl'ate beads, having an average particle size ofabout400 microns, 170 grams (34 wt. percent) of dicyclohexylphthalateplasticizer; was placed `in a Patterson-Kelley twin shell dry blenderfor 45 minutes at room temperature.

This mixture was then added to a Farrel two-roll mill (16" dia. rolls by13" long about 0.4" apart maintained at a temperature of C.). Themixture was milled for 2 minutes to obtain a viscous batch on the rolls,then 30 grams (6 wt. percent) of separated, iibrous polypropylenefiller, 0.25 inch long lloc; 6.7 grams (1.4 wt. percent) ofpolytetraiuoroethylene particles, about 0.05 micron particle size in 60%solids aqueous dispersion; and 16.5 grams (3.3 wt. percent) of acopolymer of hexauoropropylene and tetrafluoroethylene particles in 55%solids aqueous dispersion (sold by DuPont under the trade name Teon 120FEP Emulsion) was added to the batch and the milling was continued for 6minutes at a roll speed of about 35 ft./min., to ash olf the water andform a viscous, rubbery blend.

In both samples, the batch was cut from the rolls, in sheet form,rotated 90 and fed back into the nip between the hot rolls for 4two-minute periods at 135 C. and then cut from the rolls in the form ofa exible sheet about 0.04 inch thick. The use of plasticizer in Sample Eand Sample F modiiies the properties of the blend to allow use of lowerhot roll temperatures, than that used in Example 1, while providingsutlicient shear forces and fibrillation, and results in a more iiexiblesheet and a method easily adapted to a continuous process.

A The Sample E and Sample F sheets were then cooled and separatelyplaced into stainless steel molding frames about 15" x 18" x 0.050.Sample E and Sample F were then heat pressed at 150 C. and 600 p.s.i.for about 3 minutes in a moldingpress and then cooled underpressurebefore being removed.

y The Sample E and Sample F sheets were placedbetween bronze screens andimmersed in acetone for about` 16 hours to extract the solventextractable polymethylmethacrylate and plasticizer from the sheets. Thesheets were then immersed in a fresh .stirred acetone bath for twoperiods lasting 2 hours and 1 hour, removed, rinsed in deionized waterand blotted dry.

Thesamples both had goodl integrity and .tiexibility and were both insubstantially non-wettable mat form. Under v150x magnification they wereseen to comprise unmeltedv interlocked thin strands and irregular shapedparticles and larger interlocking vsupporting fibers. Porosity for-bothsamples was between about 25-50% and the pore size was about 20 microns,in diameter between the iibers and particles. Both samples vappeared tohave a ratioof about 2 strands to 8 particles (about 20% fibrillation).

' Samples E and F were tested for air permeability by the method used inExample 1. The results were as follows:

The sample E and F mats were then cnt 3.25 x 3.75" and heat and pressurebonded to separate gas diiusion electrodes by placing the cut mat on thegas entrance side of the 3.25" x 3.75" electrode and passing the mat andelectrode through heated laminating rolls at 190 C. and 300 lb./linealinch pressure. After bonding, the mat barrier layer was examined. Thefibers and particles v the mat were pressed rather than melted togetherand there appeared to be no melt ilow, the structure maintaining itsporosity and resiliency. A very good'bond was produced between thecatalytic electrode and the matbarrier layer in the nished composite. Yy l The catalytic electrode material comprisedabout 70- 80 wt.percentcarbon particles, 20-30 wt. percent l of iinely dividedpolytetrarliuoroethylcne binder andabout 0.1- milligrams of catalystpery sq. cm.v of geometric electrode area. This material was wet pastedinto lthe interstices of a fibrous, 75-95% porous, diffusion bonded,0.02 thicknicke1 plated, steel wool matrix, which" actedf as the supportfor the catalyzed active gasflayeijand asA the electrical conductor.This electrical conductor matrix comprises smooth -iibers bonded by aninterdiffusionf atoms across iiber interfa'c'zesrather thanfibers,bon'dc'e'tlV by melting, and is unique in providing large pre,volun"ies free :of metallic protrnsions and melt globules. `4It. alsoprovides a unique gas diffusion electrode structure allowing intimatecontact of electrolyte, catalystand gasQwithin theelectrical"conductor,l and a very short diffusion path for thevgas. I

Electrodes having the barrier layers of Sample E and Sample F matweretested `in the drivfeilfy vcell `shown in FIG. 4'of the drawings,vcontaining 2 7 percent KOH solution as electrolyte. They wereopferatedat 50 lina/sq. ctn.y current density, and at 125;"1 C. ,the Sample Eelectrode gave an initial voltage 4of `0.075 volt and the Sampleu Eelectrode gave an initial.,voltage of 0."108 vol-t, as measured vagainstan, Hg/Hg'| reference electrode.y Operation at 150 ina/sq. cm. ,gave apolarized voltage of `-0,.,l7l 'volt for the Sample Ev electrode and apolarized voltage of 0.255Nyolt for, .the Sample F electrode. The SampleE electrodeoperated for 13 days and the' Sample F electrodeoperated,fol-.177l days in the test cell .with no appreciable sweatingfof electrolyte through the hydrophobic membrane mat, thus EXAMPLE 4 -fA100% polyolefin mat, Sample G, comprising very high molecular weightpolyethylene was made by the following procedure. A mixture of grams (31wt. percent) of very high molecular weight polyethylene powder, havingan average particle size of between about -300 microns, a weight averagemolecular weight of about 2,000,000, and a melt ow index of about 0.01gm./10 min. (Hi-Fax 1901); 120 grams (41.5A wt. percent) ofpolymethylmethacryylate beads, having an average particle size of about400 microns and 80 grams (27.5 wt. percent of the total blend) ofdicyclohexylphthalate plasticizerl was used. This mixture was placed ina blender and mixed and then hot roll milled as in Example 3, using thesame process conditions, except that no filler fibers or fluorocarbondispersion was used in the blend. The blend was then ground in a hammermill and extruded as in Example l, using the same process conditions. Itwas then hot roll sized at C. into a layer'about 0.04 thick. The sizedlayer was cut from the roll as a sheet and placed into a stanless steelmolding frame about 6 x 6" x 0.030".

This sheet was heat and pressure molded and immersed in an acetone bathas in Example 3, using the same process conditions, to provide anon-wettable mat. 'Ihe mat had very good integrity and fair exiblity,and under magnification seemed to comprise interlocked strands andirregular shaped particles. Porosity was between about 30-50% and thepore size was about 40-60 microns. This mat appeared to have a ratio ofabout 4 strands to 6 particles (about 40% brillation). A higher degreeof fibrillation could easily be achieved by repeated extrusion althoughadding to production costs. The mat was tested for air permeability bythe method of Example 1. The results were as follows:

Air permeability Thickness (mils) (en. cm./min.)

y Before After Before After Sample comprescomprescomprescompres-(extruded) sion sion sion sion Q .-.Q V39 920 ing aan electricalconductor; the porous body consisting essentially of particles of aconducting material inert to the electrolyte and of a resinous binderinert to the electrolyte and including a catalyst; the hydrophobicbarrier layer forming an outer membrane layer on the gas entrance sidecomprising a mat of pressed, 'interlocked, brillated strands andirregular shaped particles ofvery high molecular weight polyethylenehaving a weight 'average molecular weight over 1,750,000 land a melt owindex of up to about 0.02.

`2. Thegas diffusion electrode of claim 1 wherein the resinous binder isa` hydrophobic polymer selected from the groupconsisting of polymers andcopolymers of liuorinated hydrocarbons, chloro-iiuorinated hydrocarbons,ethylene,` propylene, polysulfone and mixtures thereof, the catalyst isselected from the group consisting of at least one of the metals ofplatinum, iridium, ruthenium,

rhodium, palladium, gold, mercury and silver, the conducting materialparticles are carbonaceous particles selected from the group consistingof carbon, graphite, boron carbide and mixtures thereof and the resinousbinder varies from about 10 to '50 weight percent of the composition ofthe electrode body.

3. The gas diffusion electrode of claim 1 wherein the barrier layer matis about 2050% porous and comprises at least 15 percent brillatedstrands of very high molecular weight polyethylene.

4. The gas diiusion electrode of claim 3 wherein the electricalconductor is a 75-95 percent porous, 0.01-0.04 inch thick matrix ofdiffusion bonded metal fibers, and the conducting material particles,resinous binder and catalyst are contained within the ber matrix; thebarrier layer mat being bonded to one side of the fiber matrix.

5. The gas dilusion electrode of claim 3 wherein the fbrillated strandsof very high molecular weight po1y ethylene are between about 0.5-4000microns long and 0.005-40 microns in diameter, the irregular shaped veryhigh molecular weight polyethylene particles are about 0.05-400 micronsin diameter and the pore size between the strands and particles is fromabout 0.003 to 100 microns in diameter.

6. The gas diffusion electrode of claim S wherein the barrier layer matalso contains non-wettable thermoplastic reinforcement bers about 0.01-2inches long uniformly distributed therethrough.

References Cited UNITED STATES PATENTS 3,671,323 I 6/1972 Sandlerl136-86 D ANTHONY SKAPARS, Primary Examiner U.S. Cl. X.R. 161-158

